1. Field of the Invention
The invention relates to a method for manufacturing a field effect transistor having a widened trench, and more particularly to the method for manufacturing the field effect transistor that widens a trench thereof to form a trench bottom and a widened trench with a larger width.
2. Description of the Prior Art
With booming progress in semiconductor technology, various advanced manufacturing processes have been introducing to produce the field effect transistors, especially the metal-oxide-semiconductor field effect transistors (MOSFET).
Refer now to FIG. 1 through FIG. 3; where FIG. 1 is a schematic view of a conventional deposited polysilicon layer in the prior art, FIG. 2 is a schematic view of another conventional deposited polysilicon layer in the prior art, and FIG. 3 is a schematic view of a conventional etched polysilicon layer in the prior art. As shown, while in producing a conventional field effect transistor PA1, since a trench PA11 has a narrow width to form a structure with a high aspect ratio, so an overhang structure PA121 (as shown in FIG. 1) is easily formed during a process to form a polysilicon layer PA12. It is noted that an oversized overhang structure PA121 would lead to an early seal of the trench PA11 at the stage of forming the polysilicon layer PA12. As a result, a poly seam A (as shown in FIG. 2) would be formed. The poly seam A would pack processing gas inside the trench PA11. Hence, in the following processes, the packed gas inside the trench PA11 would be heated to diffuse and further to damage elements or to reduce liability of the chip.
In addition, as shown in FIG. 3, while in an etching process, the poly seam A would cause uneven thicknesses to the etched layers. Thus, an unstable critical voltage would be met. If a gate is energized upon existence of a poly seam A, excessive current leak is quite possible. Thus, while in producing a trench of a MOSFET, an improvement to eliminate the poly seam is definitely urgent to the art.